In the past, it was believed that sleep was simply a decrease in brain activity that occurs while awake. However, we now know that sleep is an active and highly structured process in which the brain recovers energy and rearranges memories.
The analysis of sleep is done from its division into phases, each with its distinctive characteristics. In this article we will describe the five phases of sleep, Which in turn can be divided into slow wave periods and fast wave periods, better known as “REM sleep”.
Phases and cycles of sleep
The dream was misunderstood until the middle of the twentieth century, when it began to be scientifically studied thanks to electroencephalographic activity recordings.
In 1957, physiologists and researchers William C. Dement and Nathaniel Kleitman described five phases of sleep. Its model is still in force today, although it has been modernized thanks to the development of new analysis tools.
The phases of sleep proposed by Dement and Kleitman and which we will detail in this article they continually occur while we are sleeping. Sleep is structured in cycles, that is to say in successions of phases, of approximately 90 to 110 minutes: our body goes through between four and six sleep cycles each night during which we are resting properly.
During the first half of the night, the slow phases of sleep predominate, while fast or REM sleep is more common as the night progresses. Let’s see what each of these types of sleep consists of.
Dream of slow waves or not REM
Slow sleep accounts for about 80% of total sleep. During the four phases of which it is composed, cerebral blood flow decreases compared to waking and REM sleep.
Non-REM sleep is characterized by the predominance of slow brain waves, Which indicate a decrease in electrical activity in the central nervous system.
Phase 1: numbness
Phase 1 of sleep, which represents less than 5% of total sleep, it is made up of transitional periods between waking up and sleeping. It appears not only when we fall asleep, but also between different sleep cycles.
In this phase, we gradually lose awareness of the environment. Prodromes of dream activity known as hypnagogic hallucinations appear frequently, especially in children and people with narcolepsy.
During sleep mainly alpha waves are recorded, Which also occur when we are relaxed while waking up, especially with our eyes closed. Additionally, theta waves begin to appear, indicating even greater relaxation.
So phase 1 brain activity is similar to that which occurs when we are awake, and therefore in these times it is common for us to wake up to relatively light noises, for example.
Phase 2: They are light
Light sleep follows periods of numbness. During phase 2 physiological and muscular activity decreases significantly and the disconnection with the environment intensifies, so that the sleep becomes deeper and deeper.
This is linked to the greater presence of theta waves, slower than alpha, and to the appearance of sleep spindles and K complexes; these terms describe the oscillations in brain activity that promote deep sleep, inhibiting the ability to wake up.
Sound phase 2 is the most common of the 5, Achieve about 50% of the total night’s sleep.
Phases 3 and 4: are delta or deep
In the Dementy and Kleitman model, deep sleep is made up of phases 3 and 4, although the theoretical differentiation between the two has lost popularity and today we usually talk about the two together.
Slow sleep occupies between 15 and 25% of the total; about 3 to 8% correspond to phase 3, while the remaining 10 to 15% are included in phase 4.
In these phases, delta waves predominate, Which correspond to a deeper sleep. This is why these periods are commonly referred to as “slow sleep”.
During slow sleep, physiological activity is sharply reduced, although muscle tone increases. We consider that our body rests and recovers more clearly in these phases than in the rest.
Many parasomnias are characteristic of slow sleep; in particular, during these phases most episodes of night terrors, sleepwalking, drowsiness and bedwetting occur.
Fast Wave Sleep or REM (Phase 5)
The rapid eye movements that occur during this phase give it its most famous name: MOR, or REM in English (“rapid eye movements”). Other physical signs of REM sleep are the sharp decrease in muscle tone and the increase in physiological activity, As opposed to deep sleep.
It is also known in REM phases as REM sleep because during this phase, it is difficult for us to wake up even though the predominant brain waves are beta and theta, similar to those in waking up.
This phase constitutes 20% of total sleep. The proportion and duration of REM sleep gradually increases as the night progresses; this is related to the increased presence of vivid and narrative dreams during the hours before waking up. Likewise, nightmares occur in the REM phase.
It is believed that REM sleep it is essential for brain development and the consolidation of new memories, As well as its integration with those that already existed. An argument in favor of these hypotheses is the fact that the REM phase is proportionately greater in children.